Biennial:
Occurring every two years. The Legislature meets biennially. NOTE: Biannual means "occurring twice a year."
www.leg.state.or.us/glossary.html
Plant that completes its life cycle in two years and generally flowers in the second year
naturesnaturals.co.uk/Botanicals+Glossary
Biennial means every other year. Bi-annual has been used for every two year but technically is the same as Semi-annual meaning twice per year.
"Quinquennial" means occurring every five years, OR lasting for five years.
Triennial, from latin "triennium"
The adjective is quadrennial.
Octennial
biannual
Bi-yearly
tercentennial
bidecadal
pressure
Phenomenon
The amount of times something happens within a certain period of time. In Herz, and while talking about sound, this would be the amount of waves every second.
X chromosome inactivation happens in female mammals.
Leap year
it is a a centry
...an effect. Hence the term "cause and effect."
scientific law
The term "blue moon" is used when something very rare happens, the most common usage of this term is "Once in a blue moon".
scientific law